Composite structure for use in aircraft construction



H1. 8. GEBEVUNEE ETAL COHSOSITE STRUCTURE FOR USE IN AIRCRAFT CONSTRUCTION 2 Sheets-$heet 1.

Filed Dec. 9, 1943 INVENTOR Henry Bv Gibbons BY Eugene G.Z0/dy wlwiiwfi ATTORNEY.

m .ZWfi. H. a. szamoms HAL 9 3 COMPOSITE STRUCTURE FOR USE IN AIRCRAFT CONSTRUCTION Filed Dec. 9, 1945 2 Sheets-Sheet 2 \ooo qgacteoumooo\ \sooooo u ENVENTOR Henry B. Gibbons Eugene G.Zo ldy 0 ATTORNEY,

l dteuteol Aug. 3%, "sees ooMPosITs s'm'ocwcss sou use IN AIRCRAFT coNs'mUc'noN Henry B. Gibbons, Stratford, and Eugene G. Zolcly, Ridgefield, Conn., on to United Aircraft Corporation, East Hartford, Conn., a

corporation of Delaware This invention relates to improvements in composite structures of metallic and non-metallic materials and to improved methods for making the seine; and hes particular reference to lightwelcht, meld assemblies of this type for use in aircrsft construction.

The invention has among its objects to provide an improved adhesive bond between metallic and non-metallic susfaces, between two metallic surfaces, 0:: between two non-metallic surfaces.

smother object of the invention is to provide a. unitary lambasted structure, or sandwich, inc1udlac core of non-metallic material having outer layers, or of facmg material bonded thereto that are thin relative to the thickness of the and which characterized by extremel' t strength and rigidity for its weight.

ether object the invention is to provide a, stcuctmel materiel oi the above type which can produced in shapes suitable for aircraft strucfurt'laer object of the invention i to pro vlde 522E improved aircraft surface material charby e high degree of inherent smoothness, absence of wrinkling or buckling when heavily loaded and great reduction in the amount of e. zlliery supporting structure.

A feather object of the invention is to provide on improved composite structure for utilization in tooling, such as a mold of mild curvature, character-w ed by a high degree of rigidity and lien Us in weight.

A fllfilaer object of the invention is to provide a panel structure which can be readily formed to double or compound curvatures commonly found in eiscrcft s'tmctures.

A lumber object of the invention is to provide an improved method for making 2. composite structuse which can be carried out at low tempemtwes and pz-essm-es, enabling the use of deeore materials hitherto prohibited by the tempea'svtures and pressures previously required.

A further object is generally to improve the constructloe methods of making aircraft.

These and other objects of the invention will be apparent from the following detailed description of a preferred method of constructing composite panels s 1d 2. preferred form of panel structure for deem aircraft construction, several illustra- 'tive embodiments of which are shown in the accompenylzag' drawings.

is drawings, wherein similar reference characters refer to similar part throughout the several. views.

we. 1 is a perspective view illustrating a plane sandwich constructed in accordance with the present invention.

Fig. 2 is a. similar view showing a modified construction of core member.

Fig. 3 is an exploded perspective view of a core structure for a rib in an airplane wing leading edge assembly.

Fig. 4 is a similar view showing one of the overlying plates and associated reinforcing strips for the core of Fig. 3. I

Fig. 5 is a perspective view of the assembled rib.

Fig 6 is a sectional view through an airfoil body constructed in accordance with the inventlon, the rear portion thereof being largely broken away to facilitate illustration.

7 is an enlarged detail of a joint between the front and rear sections of the airfoil body of Fig. 6.

Fig. 8 is an enlarged detail showing one com stz'uctlon for securing removable panels to the airplane frame structure.

Fig. 9 shows another construction for securing removable panels to the frame structure.

The composite panel in which the inveutlou is embodied is shown in Fig. 1 as comprising a fiat core id sheathed on opposite sides by relatively thin sheet laminations l2 and M. The core Id may comprise any suitable material such as wood or otlier cellular or fibrous material or lightweight synthetic compositions. At present it. is preferred, however, to use balsa wood, or equivalent woods. due to their extreme lightness in weight. this invention making possible the use of these desirable materials to obtain extremely rigid structures. However, specific application may dictate the use of a stronger, heavier wood or other suitable material for the core. The sheets i2 and M may be of high strength synthetic material or of steel, aluminum or magnesium to suit different conditions, but for aircraftconstruction strong alloys of aluminum are preferred. By way of example, sheets of .040 inch thickness or less generally are suitable for forming surface panels utilized in the construction of the fuselage and wings of aircraft. For the purpose of disclosing the invention, a core of balsa wood has been illustrated with sheets of aluminum bonded thereto.

In making the panel, or sandwich, of Fig. 1, the surface of the aluminum sheets I! and H which are to be bonded to the core H! are first thoroughly cleaned and then coated with a thin layer '6 of adhesive cement, which may be a phenol-formaldehyde base resin, or one containing phenol-formaldehyde resin. which possesses a strong afllnity for metal. Excellent results have been obtained with Cycleweld (-3 cement. This is a polymerizable resin adhesive sold by the Chrysler Corporation of Detroit, Michigan, which has the characteristic as set forth elsewhere in this specification of bond ng itself strongly to metal when polymerized at a temperature of approximately 325 F. for about fifteen minutes. For a complete understanding and description of Cycleweld" cement, reference is made to U. S. Patent No. 2,375,854. When the resin layers have dried, they are then polymericed at 325 F. for approximately fifteen minutes. This temperature and time for curing may be varied slightly but the figures specified have been found satisfactory for Cycleweld. Follow ng this curing step, a coating it or another adhesive cement which may be a phenol-formaldehyde base resin which possesses marked afdnity for wood and also for the previously polymerized adhesive, is applied over the first resin coating on the metal sheets. A coating iii of this second type resin adhesive is also applied to the surfaces or" the wood core in to which the sheets are to be bonded. For the second adhesive Durea' No. 12,841, a phenol-formaldehyde base resin, reduced with the proper hardening agent, or accelerator, supplied by the manufacturer has been found very satisfactory, as this cement has a strong afiinity tor both wood and Cycleweld cement. Durez No. 12,641 is a polymerizable resin described by its manufacturer, Durez Plastics and Chemicals, Inc, North Tonawanda, New York, as a high viscosity concentrated solution of a one-step thermo-setting phenol-formaldehyde resin used with an acid accelerator to obtain a proper cure and bond at a relatively low temperature. This resin has the characteristic as set forth herein of bonding itself strongly to coating of polymerized Cycleweld and also to non-metallic core materials and may be cured at low temperatures and low pressures which are not destructive to such core materials a wood, for example, at a temperature of approximately 146 1 for four hours. Durez 12,041 is believed to undergo a desirable chemieal interaction with the Cycleweld when sub- .iectecl to the following curing process.

The sandwich is assembled on a rigid mold and placed under pressure, which may be obtained negatively by placing the entire assembly in a rubber bag, which is then sealed and evacuthe sandwich being produced is to serve as a skin panel, the sheet placed against the mold should be the external skin surface. If addltlonal pressure is needed, this entire assembly may be placed in a pressure chamber or autoclave. While still being subjected to pressure the assembly is placed in curing apparatus at lei? F. for a period of four hours. Here again time and temperature specified are merely illustrative, as the time and temperature used are variables contingent on the type of construction, particularly the material comprising the core, and the loads to which it is to be subjected. also if the temperature of M0 F. specified should be too high for a particular core material, a lower temperature may be used in which case the curing time will b increased proportionately or, conversely, if the material used in the core is capable of withstanding a higher temperature. a higher temperature can be used and the curlag time shortened. For a balsa wood core a curing temperature of are F. for thirty minutes is possible without injury to the wood.

t is during this curing rocess of the assemill bled structure that chemical interaction is believed to take place between the two resins, one having a specific aflinity for metal and the other having a specific affinity for wood and for the first resin, which results in a permanent bond between the metal and wood surfaces of superior strength, and between adjacent metal surfaces where splices or reinforcements are incorporated in the face sheets. Irrespective of the nature of the interaction between the two resins, there is a definite blending together of these two resins prior to setting so that when they are set, an especially strong bond occurs between them, This increase in strength of the bond is believed to be obtained through the polymerization of the Cycleweld with the second resin. When the assembled structure is removed from the curing apparatus, a further curing or setting period of approximately twelve hours, with pressure relieved and at the above mentioned temperature. may be used when it is desired to develop the extremely high strengths'which are inherent in the structure. During this setting period the standwich structure can be further fabricated for eventual incorporation into finished aircraft assemblies.

The Fig. 2 construction differs from that of Fig. l in having the core Ma formed of a fibrous material which has its grain substantially perpendicular to the respective longitudinal faces of the metal plates i2 and M, the method of bonding the core to th metal sheets being the same as that above described in connection with Fig. 1. The arrangement of the grain of the core material perpendicular to the respective longitudinal faces of the plates is the invention of Henry B. Gibbons and is described more fully and claimed in a co-pending application, Serial No. 513,619, filed December 9, 1943.

in Figs. 3, t and 5 there is illustrated one use of the sandwich structure of Fig. 1, wherein the structure is shown greatly exaggerated, better to illustrate the invention. In practice, however, the thickness of the entire sandwich may be as little as one-eighth of an inch. Fig. 5 illustrates as an example, a rib for the leading edge of an airplane wing structure and includes a core sheet generally indicated at it havin its upper and lower margins shaped to conform to the curve of the airfoil section. The elements of the core are shown in Fig. 3 and comprise a thin sheet of balsa 228 having thin marginal strips 28, 29, Bit, 3i and 32 of spruce, or other suitable material, which lie in the sarneplanes as sheet 26 with their edges in abutting relation therewith. Preferably these marginal strips are adhesively secured to the balsa sheet. A reinforcing member 3t of spruce is provided for the aperture in the balsa sheet and is also adapted to be bonded to the margins 36 of the aperture so as to lie in the same planes as the sheet 26, It will also be noted that the reinforcing member 36 has a key portion 38 which lies between the marginal strips 35 and 32 and flush with the latter to provide continuous reinforcement at the rear edge of the core.

Metal plates it of aluminum are bonded to the opposite sides of the core 24, the method of bonding the plates to the core being identical with that described in connection with Fig. 1. It will be noted that the plates to overlie the sheet 26 and the reinforcing elements surrounding the margin thereof and are co-extensive with the entire core sheet 26. Metal strips are provided on the outside iace of the plates which are generally co-exasrasse tensive with the spruce strips fill, it, ill, 3i and 22 on the core structure and are adapted to overlie the same with the metal sheet d9 interposed therebetween. Thus, the metal strips s2 and i l overlie the corresponding spruce strips 28 and 30 While the metal strip 33 overlies the spruce strip 29. A single metal strip 68 overlies the spruce strips 3i and 32 as well as the interposed key 38.

It will thus be evident that when fastening elements lltl, which may be in the form of rivets or bolts. are used as shown Fig. 5 in the marginal portions of the assembly, these fastening elements pass through the marginal spruce strips which are stronger in compression than balsa wood. is nose plate lid of metal having a lateral flange 55". n secured through fastening elements at to the leading edge of the rib, and upper and lower angle members so and Ed are secured along the upper and lower margins of the rib structure in a similar manner, the lateral flanges 59 of members and 53 matching the flange 53 to provide a sn'iooth'surface to support the surface panels of the airfoil.

Une construction for such surface airfoil panels is shown Fig. 6 in which a wing to is shown hICi-WlS section. As shown in this figure, the core 552 is formed of balsa wood with the grain of the wood running normal to the surface of the wing. The wing is formed of two generally U- shaped complements); shells Kid and 66 having their meeting edges joined as shown more in detail in Fig; I wherein epanwise metal plates 68 and are adbesively secured, by the above procto the under sides of the outer plates 12, it and the inner plates 56 of the respective shells. t process, as described ebove in connection to mg. l, the t?ycleweld" adhesive is first apll all s fifaces of the metal plates 58 and e joined and this coating is polymerized F. for approximately minutes. These SU'LLF-A'JGS the abutting surfaces of the plates of sheets to and oil which have similarly treated, as well as the e core which abut and which are then coated with Durez" No. lve as previously described. The wine section is then cured at the pre viously specified low temperatures and pressures, for example, for for four hours at atmospheric or. to sections are such as to require it, suitably higher pressures which are not destructive to the balsa wood core. In the embodiment shown in Figs. 6 and 7 the surface plates Ni one it, lit including the connect. g members forming a part thereof made of motel but as previously stated the sol-"ace plates of the sandwich may be formed of syn tic r 3 shows one means of securing removable sandwich panels to a similar panel 82 comprising a part of the framework of the airplane structure. Special angle shapes as and so ere permanently bonded to the panel 82 by the method previously described in connection with l for bonding the lamination ill to the core The legs 23% and till of these angle shapes which lie parallel to the planes of the panels to have the panels secured removably thereto by flush type screws 32 with pass through the panels or re threaded into the less so and till.

ll another construction is shown in which removable panels have threaded inserts cdliesively secured in recesses in the under surfaces thereof preferably prior to bondlng the plates to thereto. Suitable cap screws We pass through the horizontal legs of angles it: and are screw threaded into'said inserts to secure the panels 93 detachably in place on the sandwich type panel m4 which may be, for example, a bulkhead. The vertical legs of angles I02 maybe adhesiveiy bonded to panel I04 as in the Fig. 8 constructioncr may be secured by rivets I03, as shown in Fig. 9.

In making the various structures comprising the surface panels and supporting framework thereof encountered in aircraft construction, the assemblies consisting of flat panels are preferably made as previously described in connection with Figs. 1 and 2. when the surfaces to be formed are curved, the same method can be followed it the curves are not too severe. For example, mild curves within the bending allowances of the materials comprising the sandwich can be produced by assembling the sandwich as a flat panel and placing it on a rigid mold having the desired shape prior to enclosing the assembly in the evacuated bag for the curing process. In this case, atmospheric pressure may be sufficient to cause the panel to conform to the mold. Greater pressure, as obtained in an autoclave. may allow a higher degree of forming than just atmospheric pressure. When compound or severe curves are present,.the core and surface sheets are pre-cut and pro-formed. The metal sheets are then coated with resin, cured'as previously described, and the coatings of the second resin are applied, after which the assembly is placed in a jig, or form, forthe curing step. In some instances, it may be desirable to assemble the pro-cut and preformed elements of the sandwich in male and fe male dies before the resins set and apply pressure on the dies during the setting process, or it may be desirable to assemble the pro-cut and pro-formed elements of the sandwich on a rigid mold, a method described above in relation to panels of mild curvature. In any event, the final curing process is carried out at the relatively low temperature and pressure specified as contrasted with the high temperatures and pressures hereto required by others to obtain strength approaching the results obtained by this invention.

While atmospheric pressure or less and curing temperatures of 140 F. results, in some instances where the material of the sandwich will permit it,'temperatures as high as 212 F. and pressures as high as pounds per square inch may be used to save time. However, a curing temperature of 212 F. and a pressure of 100 pounds per square inch is low when compared with prior methods of making adhesive bonds of comparative strength.

When molds are required for the production of curved surfaces, the molds can advantageously be made of a sandwich of wood and metal of the required shape, to produce a rigid mold of low cost which is very light in weight even for molds of very large size.

It will be evident that asa result of this invention an improved method has been provided for obtaining a superior bond between metallic surfaces, metallic and non-metallic surfaces, or between two non-metallic surfaces, without requiring the use of high temperatures and pressures.

It will also be evident that as a result of this invention a superior method has been provided for bonding metal and wood laminations by which an improved composite structure of this type is provided. Further, by reason of the use of two different resins, one of which has an ator under give very good asrasse finity for metal and is cured prior to the assembiy of the metal sheets with the wood core, and by the use of pressures which are low as contrusted with those formerly required, the use of light-weight, low density core materials for such composite panels has been greatly extended. For example, the 325 F. required to cure the Cycleweld cement coating on the facing sheet is abovethe desirable temperature for certain very desirable, light-weight core materials, but by the coating applied to the facing sheet prior to assembly with the core, the final curing can be carried out at a temperature well below the critical temperature for the core material, 1. e. the temperature at which the material will be damaged. Similarly, pressures formerly required with cold setting adhesives, and pressures of or above 300 pounds per square inch required at to 356 F. for thermo-setting adhesives are avoided by the process of the present invention, thus, for the first time, making it possible to use certain core materials, such as balsa, which will not withstand these high pressures.

It has been found in bonding together matewith different rates of thermal expansion in which the curing is carried on at the elevated temperature of, for example, 300 F., that severe stresses are placed on the bond when reduced to normal temperatures. The process of the present invention overcomes these difficulties, since the curing of the sandwich is carried out at relatively low temperature, which avoids such stresses.

will thus be evident that the present invention not only greatly extends the use of commsite structures for airplane manufacture, but bles use of especially light-weight core rials heretofore impossible with the previprocesses manufacture of such structures, while obtaining many times the local rigidity and strength oi an equivalent weight oi aluminum alloy required present airplane constructions. By reason of the simplicity oi the improved procas contrasted with the conventional aircraft fabrication method, a construction is produced which is markedly less expensive. For example, the number of wing ribs required in wings made in accordance with the present invention may be approximately one-third the number required in all metal wing construction, thus not only greatly simplifying the work of constructing the wing out greatly reducing its cost and its weight.

Further, by utilizing this light weight, rigid sandwich construction in tooling for the production of aircraft, marked savings can be effected while producing molds which are readily handled even in the case of molds for the production of extensive surface panels for Wing and fuselage structures.

The present invention is also particularly welladaptcd for mass production or" aircraft and results in a construction far superior to the present all-metal construction.

While the invention has been described particularly in connection with aircraft construction. it will be evident that the invention is not limited to this field but has many applications in other arts in which a light-weight, rigid sandwich type structure would be advantageous. It will further be understood that the particular lil temperatures and pressures specified are for illustrative purposes only and that the same may vary widely to meet different conditions encountered, and that the invention is limited in its scope only by the claims appendant hereto.

Having now described the invention so that others skilled in the art may clearly understand the same, what it is desired to secure by Letters Patent is as follows:

1. A body comprising complemental sections, each comprising a laminated shell including a core of light cellular material and inner and outer plates of metal permanently bonded thereto, and means for connecting the abutting edges of said shells including a metal strip extending along said abutting edges on the inner face of and overlapping the abutting plates thereof and adhesively bonded thereto.

2. A body comprising complemental sections, each comprising a laminated shell including a core of light cellular material and inner and outer plates of metal permanently bonded thereto, and means for connecting the abutting edges of said shells including metal strips extending along said abutting edges on the inner faces of and overlapping the abutting plates thereof and adhesively bonded thereto through the interaction of two phenol-formaldehyde base resins polymerized through the application of low pressures and low temperatures.

HENRY B. GIBEONS. EUGENE G. ZOLDY.

resrsanuons orrnn The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 785,521 Roach Mar. 21, 1905 1,019,407 Baekeland Mar. 5, 1912 1,077,206 Maurer Oct. 28, 1913 1,393,143 Novotny Nov. 22, 1921 1,549,320 Lundin Aug. 11, 1925 1,559,846 Brown Nov. 3, 1925 1,565,532 Tupper Dec. 15, 1925 1,634,809 Weiss July 5, 1927 1,771,567 Auld July 29, 1930 1,954,130 Higgins Apr. 10, 1934. 1,972,397 Loetscher Sept. 4, 1934- 2,015,396 Menger Oct. 1, 1935 2,062,088 Dreyer Nov. 24, 1936 2,117,085 Ensminger May 10, 1933 2,180,934 Dent et al Nov. 21, 1939 2,233,875 Schmidtet al Mar. 4, 1941 2,304,718 Swart Dec. 8, 1942 2,306,295 Casto Dec. 22, 1942 2,317,364 De Bruyne et a1 Apr. 27, 1913 2,376,854 Saunders et al May 22, 19 15 FOREIGN PATENTS Number Country Date 145,123 Great Britain June 21, 1929 453,648 Great Britain Sept. 14, 1936 OTHER REFERENCES Cycleweld article published in Modern Plastics of September 1943, pages 65-69 and 152, (Copy in Division 33.) 

